Online classifieds are a favorite place for scammers to play their tricks and target their victims. They often hijack the listings for legitimate rental properties and make themselves the contact person. Their goal is to get you to pay the full rental or at least a sizeable security deposit upfront.

"We've seen horror stories where people wired the money, thousands of dollars, got some bogus keys, showed up and the rental property wasn't there—it didn't even exist," Quinlan told me. "Just put yourself in their position. It makes your stomach turn."

Earl and Barbara Renken, who live near Sacramento, California, got burned a few years ago. They had gone online and found a summer rental apartment in Anchorage, Alaska, at a low price. They corresponded with the sales agent for the rental company who was so accommodating, he even asked them what color they wanted him to repaint the place. The Renkens' son, who lives in Alaska, drove by the property and said it looked good.

When they arrived in Anchorage, they discovered that they'd been duped. They had paid almost $1,500 to an online swindler. The landlord told the Renkens the place wasn't for rent and hadn't been advertised.

"I was really ticked off," Barbara remembered.

Earl contacted his credit card company and the charges were reversed. Luckily, they had a friend in town who let them spend the summer for free in his basement apartment.

"If we'd been strangers in town, just wanting to summer in Alaska, we would have been up the creek," Earl told CNBC. "After paying for this place, we couldn't have afforded to pay for another, so I don't know what we would have done."

Rental scams can happen anywhere, but properties in popular vacation destinations, such as Florida, are a favorite target.

In Cape Coral—just south of Fort Myers—the police department has 20 active vacation rental scam cases. Some of the victims who've fallen for these bogus ads have lost several thousand dollars.

"Once that wire transfer is completed, it's almost impossible for us to track it down," said Detective Sgt. Dana Coston. "In most of these cases, the price that was offered was very far outside the market norm, and that should be a red flag."

These scams can also take a toll on the people who own a home or apartment and want to make some money by renting it.

Barbara Harms, who lives in Maryland, owns a vacation rental in Fort Myers Beach, Florida. Earlier this month, Harms received a call from a woman who had seen an ad for her property on Craigslist. The woman had been emailing a man who said he owned the property and wanted her to wire money to his bank account. The woman told Harms she had the feeling it was a scam. It was.

"They took our entire ad from HomeAway, including the wording that describes our place as well as all of our pictures, even the pictures of local attractions and put it on Craigslist," Harms told CNBC. "It just blows me away that someone could do this to a family going on vacation."

The would-be crooks did make one big change—the price. It went from the low-season rate of $300 a night to just $200.

Don't become a victim

Most vacation rentals go as planned, but you've got to do your homework. Does the property and rental company exist? Are there customer reviews? See if the Better Business Bureau or the local chamber of commerce has any information.

Avoid anyone who wants you to wire them money. Pay with a credit card. If there's a problem, you can challenge the charge.

"Check and double-check and vet these things ahead of time," said Mike Scott, the sheriff in Lee County, Florida. "Make sure who you're dealing with and what you're dealing with in order to avoid becoming a victim."